Nanostructured material performance in corrosive environments (Corrosion x nanostructured materials) Zehbour Panossian

Transcription

1 Nanostructured material performance in corrosive environments (Corrosion x nanostructured materials) Zehbour Panossian

2 Outline Brief concept of corrosion. The four traditional forms of corrosion protection. Corrosion protection through nanostructured materials (parallelism with traditional concepts) Final remarks

3 Corrosion definition Transformation of a metal in a metallic ion through its interaction with the species in the exposed environment. METAL Environment CORRODED METAL Fe Zn Al Fe 2+ Zn 2+ Al 3+

4 Corrosion process continuity... No interaction No corrosion Immunity Very rare Fe 2+ Fe 2+ Fe 2+ Fe 2+ Fe 2+ Fe 2+ Active metal Metal or alloy cannot be used in the considered environment Metal with corrosion product layer Good performance or localized corrosion

5 The four practices of corrosion protection select another metal or alloy or modify the composition or the microstructure of the metal or alloy: materials science interpose a barrier between the metal and the environment: metallic and organic (paint) coatings; condition the environment: by adding corrosion inhibitors or oxygen scavengers or by deairating; cathodic or anodic protection: by modifying the potential differences across the metal and the environment.

6 Nanocrystalline (nc) x microcrystalline (mc) The use of nc materials is being developed for different purposes, especially due to the physical and mechanical properties such as wear resistance, smoothness and brightness. nc materials present a higher density of microstructural flaws : the intercrystalline (intergranular) paths increase up to almost 50 % when the grain size decreases from 1 µm to 5 nm. The classical principle states that the intergranular paths present a higher energy than the bulk grain due to the presence of voids, impurities, segregations. Thus, chemical reactions and diffusing processes are enhanced within grain boundaries.

7 Nanocrystalline nickel nc ultrapure Ni is produced by electrodeposition. This material is preferred when better physical and mechanical properties are required: Hardness Wear resistance Friction nc is 5 times harder than mc nc is 170 times more wear resistant than mc nc presents ½ of friction coefficient value than mc How about the corrosion resistance? Studies in H 2 SO 4 Nanocrystalline (nc) electrodeposited - 32 nm; Microcrystalline (mc) 90 % cold rolled and full annealed 100 µm MISHRA, R.; BALASUBRAMANIAN, R. Effect of nanocrystalline grain size on the electrochemical and corrosion behavior of nickel. Corrosion Science, 46 (2004, ) ROFAGHA, R. et al; The corrosion behavior of nanocrystalline nickel. Scripta Mettallurgica et Marterialia, 25 (1991)

8 After electrochemical tests Ni nc Uniform attack (necked eye) Ni mc Localized attack Images after anodic polarization In classical theory, the performance of Ni in diluted H 2 SO 4 depends on the characteristics of the passive film. The film breakdown occurs within intergranular paths. Thus, the increase of intergranular paths of the nc Ni increases the corrosion susceptibility of nc Ni in comparison to the mc Ni. nc presents better physical and mechanical properties, but is more prone to corrosion.

9 Fe8Al nanocrystalline alloy Thin films (350 nm) of nanocrystalline Fe alloyed with 8 % de Al were applied on inert substrates (quartz, graphite and gold). The grain size of the alloy was 40 nm. The corrosion behavior of nc alloy was compared to a mc alloy with the same composition. The grain size of the mc alloy was in the range between 200 µm and 300 µm. The corrosion tests in 0,1 mol/l Na 2 SO 4 solution, with: ph = 1. At this ph, a passive film does not form on the alloy which presents an active dissolution, as the formed ferrous ions remain dissolved in the acid. The corrosion reactions are: Me Me n+ + ze - 2H + + 2e H 2 Active dissolution ph = 6. At this ph, a passive film, constituted by aluminum oxide, is formed on the alloy surface. Thus, the performance of this alloy will depend on the maintenance of the integrity of the passive film. The alloy may suffer localized corrosion (pits ). The corrosion reactions, when occur, will be: Visão clássica (mc) Me Me n+ + ze - 2H 2 O + O 2 + 4e 2OH - Pit formation ZEIGER, W. et al., Corrosion behaviour or nanocrystalline Fe8Al alloy. Nanostructered Materials. 6 (1995)

10 Fe8Al nanocrystalline alloy Results: ph = 1: greater corrosion of the nc alloy was observed. Classical behavior was observed: the corrosion was more intense in nc alloy due to its higher intergranular paths. ph = 6: less corrosion of the nc alloy was observed. Once again the classical behavior was observed: at this ph, a passive film of aluminum oxide was formed. The film formed on nc alloy was more effective than the film formed on mc alloy. This later behavior can be explained as follows

11 Fe8Al nanocrystalline alloy At the earlier stage of the formation of Al 2 O 3 This region becomes impoverished in Al Diffusion of Al occurs due to a gradient in the Al content between the region beneath the Al-oxide and the bulk alloy. These diffusion preferentially through the grain boundaries. The diffusion is enhanced in nc alloy due to its higher density of intergranular paths. As in nc alloy, the protective Al-oxide is formed rapidly, the uniformity and the compactness of passive film is enhanced. This film constitutes an effective barrier and protects the alloy against corrosion. It is possible to apply Fe8Al alloy films on carbon steel surface as a protective measure.

12 High temperature oxidation (> 650 o C) nc alloy coating nc thin films High temperature resistant alloys normally contain Ni, Cr and Al. The oxidation resistance of these alloys depends mainly on the formation and preservation of an oxide layer constituted mainly by Cr 2 O 3 and/or Al 2 O 3.. Large amount of Cr and Al are required. Al impairs the mechanical properties of these alloys. The formation and preservation of the oxide film is diffusion dependent (as previously mentioned). nc high temperature resistant alloys: the amount of Al required is lower in nc alloy because the rate of Al diffusion is enhanced due to high density of grain boundaries. This behavior permits a formation of more uniform, adherent and compact oxide layer. For example: in Ni-Cr mc alloys, 8 % of Al is required whereas in Ni-Cr nc,only 2 % of Al guarantees a good performance. mc (370 nm) nc 60 nm Ni-20Cr-2Al after exposure at 1000 o C for 100 h GAO, WEI; LI, ZHENGWEI. Nano-structured alloy and composite coatings for high temperature applications. Materials research. 7(1): , 2006

13 The four practices of corrosion protection select another metal or alloy or modify the composition or the microstructure of the metal or alloy: materials science interpose a barrier between the metal and the environment: metallic and organic (paint) coatings; condition the environment: by adding corrosion inhibitors or oxygen scavengers or by deairating; cathodic or anodic protection: by modifying the potential differences between the metal and the environment.

14 Organic coating The painting of metallic materials is a widely adopted practice. The performance depend on: nature of the resin: alkyd, epoxy, phenolic, etc..., nature of the pigment: for example, to improve anticorrosive properties surface treatment: to improve the coating adhesion

15 Ceramic nanoparticles Nanoparticles of TiO 2, SiC, Al 2 O 3 (pigments) are incorporated into organic coatings in order to enhance the barrier properties and, thus, to improve the corrosion resistance.

16 Ceramic nanoparticles The use of nanoparticles as pigments in paints There is a significant improvement of the paint performance: better corrosion resistance, gloss retention, better resistance to mechanical damages (the nanoparticles are hard)!

17 The four practices of corrosion protection select another metal or alloy or modify the composition or the microstructure of the metal or alloy: materials science interpose a barrier between the metal and the environment: metallic and organic (paint) coatings; condition the environment: by adding corrosion inhibitors or oxygen scavengers or by deairating; cathodic or anodic protection: by modifying the potential differences between the metal and the environment.

18 Organic coating The painting of metallic materials is a practice widely adopted. Their performance depend on: nature of the resin: alkyd, epoxy, phenolic, etc..., nature of the pigment: for example to improve anticorrosive properties surface treatment: to improve the coating adhesion Fosfatization with Cr (steel) Alodine- fosfocromatization (Al) Adhesion promoter prime coatings

19 Corrosion inhibitors Smart coatings: chromate coated galvanized steel - when requested (moisture condenses on the surface), the system responds to protect the substrate NaCl NaCl + chromate Chromate is good but toxic

20 Smart coatings Thin films containing TiO 2 nanoparticles applied on Al and Al alloys there are many attempts to apply this type of films on Al and Al alloys aiming to replace the smart chromate conversion coatings. But the obtained performance is not good: even with a great effort attempting to avoid the formation of pores and cracks, the corrosion protection performance is low because the film acts only as a barrier; the solution for this problem was the development of new technology: to store an inhibitor in the nanoparticles which is released when requested. Ceasing the stimulus, ceases release It took some time to imitate chromate conversion coating behavior which has been in use for a long time...

21 Smart coatings Thin films with TiO 2 nanoparticles applied on Al do not present a good performance: Primer

22 Smart coatings OH - OH - OH - OH - OH - The alkalinization release inhibitor.. Al Al e- 2H 2 O + O 2 + 4e - 2OH -

23 Smart coatings The alkalinization release inhibitor.. Al Al e- 2H 2 O + O 2 + 4e - 2OH -

24 The four practices of corrosion protection select another metal or alloy or modify the composition or the microstructure of the metal or alloy: materials science; interpose a barrier between the metal and the environment: metallic and organic (paint) coatings; condition the environment: by adding corrosion inhibitors or oxygen scavengers or by deairating; cathodic or anodic protection: by modifying the potential differences between the metal and the environment.

25 Cathodic protection A natural potential is established across the metal environment interface which is known as CORROSION POTENTIAL or OPEN CURCUIT POTENTIAL In general, corrosion potential is higher than the equilibrium potential. Me n+ + ne - corrosion Me Me n+ + ne - Me Cathodic protection is a protection technique in which the metal/environment potential is lowered by: Me n+ + ne - protection Me using an electric current; connecting it to another less noble metal.

26 Anodic protection A natural potential is established across the metal environment interface which is known as CORROSION POTENTIAL In general, corrosion potential is higher than the equilibrium potential. Me n+ + ne - Me corrosão Me n+ + ne - Me Anodic protection (applied only to metals which undergo passivation): is a protection technique in which the metal/environment potential is increased in order to enhance the protective characteristics of the passive film. Me n+ + ne - proteção Me

27 Ceramic nanoparticles incorporation in sol-gel films Thin films with ceramic-tio 2 nanoparticles applied on AISI 316 stainless steel the protection mechanism is different in dark and light conditions: In the absence of light: the TiO 2 coating acts as an effective barrier. Experiences show that, in a dark condition, 40-nm of TiO 2 nanoparticles, incorporated in a 400-nm sol-gel film, are able to reduce the corrosion current in three order of magnitude in chloride-containing solutions; In the presence of light: photoelectrons are generated in the conduction bands of the TiO 2 nanoparticles. These electrons are transferred to the stainless steel substrate and cause a decrease of the open circuit potential. In this situation, the stainless steel is subjected to cathodic protection. SHEN, G.X.; CHEN, Y.C.; LIN, C.J. Corrosion protection of 316 L stainless steel by TiO 2 nanoparticles coating prepared by sol-gel method. Thin Solid Film,489: , 2005

28 Conductive coatings Conductive polymers (polyaniline, polytiofene and polypirrol) have been used as protective coating of carbon steel due to their special electrochemical characteristics and their electronic and ionic conductive capacity. These polymers are able to increase the open circuit potential up to more positive levels, in the range of passivation. This behavior protects the substrate through anodic protection mechanism: a compact and adhered oxide layer is formed between the interface metal/polymer which acts as an effective corrosion protection barrier. Compact and adhered oxide layer Carbon steel coated with polyaniline ROUT, T.K.et al. Development of conducting polyaniline coating: a novel approach to superior corrosion resistance. Surface and Coating Thechnology, 167: 16-24, 2003

29 Conductive coatings It is possible to incorporate in the conductive coatings organic or inorganic nanoparticles, with oxidizing properties, such as MnO 2, V 2 O 5,Fe 2 O 3, Fe 3 O 4. This practice guarantees the oxidizing capacity (to shift open circuit potential to more positive values) and thus, improve the corrosion resistance capacity of the conductive coatings.

30 Conductive coatings Tests were conducted : Conductive films obtained with polypirrol (elecodeposition using a rotating electrode): Polypirrol+ Fe 3 O 4 (particle size from 200 nm to 300 nm, clusters ~1 µm), Polypirrol+ Fe 3 O 4 (particle size from 200 nm to 300 nm, clusters ~1 µm Immersion tests in NaCl solution showed that the oxidizing capacity of the conductive polymer increases in the presence of oxides, and this increase was greater for Fe 3 O 4. This ensures a more effective surface passivation of the steel substrate, improving the efficiency of anodic protection

31 Final remarks The classical corrosion and corrosion protection principles seem to be valid for nanostructured materials. The use of nanostructured material is already a reality and it should be widespread soon. Attention: in all consultant literature, the corrosion studies were conducted through electrochemical tests (polarization curves, linear polarization and electrochemical impedance spectroscopy) and accelerated corrosion tests (such as salt spray). In the eighties of the past century, coatings, known as new generation coatings, were launched into the automotive industry as presenting excellent corrosion resistant properties. The corrosion resistance of these coatings were evaluated through laboratory tests similar to those of nanostructured coatings. Huge amounts of money were lost... The characterization techniques of nanostructured materials are at a very advanced stage but the corrosion resistance behavior of evaluation tests must be improved.

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